This invention relates to a novel combination of materials for use as wear-resistant bearings, in particular, wear-resistant bearings used in artificial heart pumps.
Bearings comprise two contacting surfaces, which support varying loads while allowing relative motion between the contacting surfaces. Typical bearings include ball and other rolling-element bearings as well as journal-type bearings. Many bearings utilize lubricants, which cushion one surface from another when there is relative motion between the two surfaces. In particular, if two surfaces move relative to one another and fluid is introduced between them, a pressure gradient forms between the two surfaces preventing surface to surface contact and significantly reducing friction. For example, in a ball bearing, the ball typically rolls on a cushion of lubricant.
Reducing friction in lubricated bearings requires the following conditions. First, the relative velocity between the two surfaces must exceed a threshold velocity. Second, a sufficient volume of lubricant must be present. Third, surfaces must be smooth enough so that any surface roughness or asperities have heights that are less than the thickness of the lubricant. Even under these conditions, surfaces still come into contact from time to time and friction is increased. This can especially occur during heavy loading or irregular velocity.
When the two surfaces that move relative to one another come into contact, a phenomenon known as cold welding can occur. In cold welding, surface asperities from one surface weld to the other surface, and as the surfaces move, the welded asperities are pulled from their original surface forming pits in the base metal of that surface. Most of the welding asperities will separate from both surfaces becoming wear particles, which contaminate the lubricant. As the lubricant breaks down due to contamination, friction between the surfaces increases, thereby accelerating the production of wear particles and further breakdown.
It is often desirable to reduce wear in bearings and consequently increase the longevity of the bearings. One approach to reducing wear is the use of dissimilar materials for different surfaces of the bearing, reducing wear associated with cold welding. Another approach to reducing wear is the ion-implantation of one or both of the bearing surfaces. This approach is not completely understood, but it reduces friction between bearing surfaces in some cases. Nonetheless, even with these approaches, the wear properties of bearings vary widely depending on the particular surface materials and treatments.